This invention relates to free standing stoves in which heat from the stove may be used to heat the air in a room and/or to cook food. Such stoves are traditionally designed to use wood or coal as fuel, although other combustible solids may alternatively be used.
Free standing wood and coal burning stoves were used for many years prior to the advent of electricity and the widespread use of fuel oil and natural gas. The term "free standing" as used herein is intended to define the type of stove which is complete in and of itself. For example, it need not necessarily be positioned within or be used in combination with any other type of stove or fireplace. It may be advantageous in some instances, however, to utilize a fireplace flue stack in the event the stove is going to be positioned in the vicinity of a fireplace.
Stoves of this type are intended to burn the fuel as completely as possible and transmit the heat released thereby into the room or dwelling. Early stoves were often constructed of cast iron or other metals which were capable of absorbing and subsequently releasing large amounts of heat. Wood and coal burning stoves have thus historically relied primarily upon radiation of heat from the stove into the immediately surrounding air space. While such stoves were acceptable to the early pioneers who lived in small dwellings and had abundant, free fuel close at hand, shortcomings became apparent as civilization and technology progressed. Specifically, the inefficient design of early stoves resulted in most of the heat passing out through the flue stack.
Realization of the inefficiency of the early stoves led to the development of more advanced units which took advantage of natural convection of air and combustion gasses through the stove. An example of this type of stove is disclosed in U.S. Pat. No. 4,127,100. This patent describes a stove having the conventional box-like shape fabricated from iron or steel plate. The stove includes a single combustion chamber or firebox in which wood is placed for burning. Draft inlets are positioned in the front of the firebox while the flue or stack is at the rear. An air duct is located toward the back of the firebox. The lower end of this air duct receives ambient air and directs it upwardly and then across the top of the firebox in a plurality of tubes before discharging the air out the front of the stove. The gasses of combustion thus contact these air tubes before they exit out the rear of the firebox. As the hot gasses pass out of the combustion chamber they are directed downwardly by a baffle plate which causes the gasses to contact the upwardly extending air duct, thus imparting heat to the air as it enters the air duct.
This design is typical of second generation wood burning stoves in that it attempts to utilize the natural convection of air and gasses through the stove. While this stove is an improvement over first generation designs, it is lacking in several respects. First, in this stove the greatest amount of combustion chamber heat is applied to the air tubes at the rear of the firebox where the ambient air is relatively cool, rather than toward the front of the firebox where the ambient air has reached its highest temperature. A second disadvantage with this stove is that means are not provided for preheating ambient air which is induced into the firebox. Perhaps this is one reason why the draft inlets are positioned in the front rather than at the bottom of the stove, thereby ensuring that the warm, rather than cool, air will be induced into the stove. However, this removal of warm air from the space to be heated defeats the purpose of even having a stove. Moreover, induction from the front provides for poor combustion of the logs in the firebox.
This last point is an important one, not only in order to economize on fuel, but also to reduce the existence of particulate and other visible emissions passing up the flue stack, which not only pollute the air but also result in soot and/or creosol buildup in the flue stack. One way to reduce these emissions while obtaining the greatest amount of energy from a given amount of fuel is to use a plurality of combustion chambers. However, multiple-chambered stoves have met with little success. One reason for the failure of such stoves heretofore is that they have not justified their additional expense with a corresponding increase in efficiency.
It is a primary object of the present invention to provide an improved free standing stove which effectively and reliably overcomes the aforementioned drawbacks and limitations of the prior art proposals.
This invention responds to the problems presented in the prior art by providing a stove with a firebox or first combustion chamber for initiating the combustion of fuel, and a secondary combustion chamber for continuing the combustion. First and second conduits are provided for conveying air to the firebox and the secondary combustion chamber, respectively. A third conduit conveys the gasses of combustion from the secondary combustion chamber.
In one preferred embodiment a draft inlet damper is included to control the flow of air into the firebox, and the second conduit is mounted to receive the air from the first conduit which bypasses the firebox. In another preferred embodiment a fourth conduit is mounted in thermal contact with the third conduit for at least a substantial portion of the length of the third conduit. The phrase "thermal contact" as used herein is intended to define a relative position between the third and fourth conduits such that heat will be conveyed from the gas in the third conduit to the air in the fourth conduit, thus ensuring that the air in the fourth conduit will be heated as it passes through the stove.
These and other objects, features and advantages of the present invention will be apparent from the following description, appended claims and annexed drawings.